<FluentCalendar DisabledDateFunc="@DisabledDate" @bind-Value="@SelectedValue">
    <DaysTemplate>
        @if (!context.IsInactive &&
            (context.Date.Day == 5 || context.Date.Day == 15))
        {
            <div style="color: red; font-weight: bold;">
                @context.DayNumber
            </div>
        }
        else
        {
            @context.DayNumber
        }
    </DaysTemplate>
</FluentCalendar>
<p>Selected date @(SelectedValue?.ToString("yyyy-MM-dd"))</p>

@code
{
    private DateTime? SelectedValue = null;

    private bool DisabledDate(DateTime date) 
    {
        return (date.Day == 3 ||
                date.Day == 8 ||
                date.Day == 20);
    }
}
